X-ray shadow microscope



April 17, 1962 J. B. LE POOLE x-RAY sHADow MIcRoscoPE Filed July 24, 1959 INVENTOR Jan bart le pol United States An X-ray shadow microscope is a device comprising an X-ray tube in which the electrons are concentrated into a very small focal spot. An adjustable electron-optical system serves to concentrate the beam of electrons.

yIt has previously been suggested to receive an electronoptical image of the focal spot on a fiuorescent screen arranged on the cathode side of the electron-optical system, the screen being struck by electrons emerging from the focal spot, which electrons are led by said system or part of the stages constituting the system towards the fluorescent screen. The adjustment of the electron-optical systern which provides maximum definition of the focal spot is found by means of the visible image on the fluorescent screen.

However, if the electron-optical system has optical errors, as is usually the case, the size of the focal spot found with the aid of the image on the fluorescent screen has not yet been reduced to the extreme minimum.

In electron microscopes, it is possible to correct errors of a lens, more particularly the astigmatism of an electron lens, due to vthe fact that the spot of the beam is visible and the influence of the correction is recognizable by different characteristics of the spot. The expectation that the astigmatism of the electron-optical system in the X-ray tube of an X-ray shadow microscope can be shown in the image of the focal spot produced on the fluorescent screen has been found not to be fulfilled. The electronoptical image of the focal spot is evidently the smallest stray figure which, as is well-known, has rotational symmetry for astigmatic lenses.

The invention purports to obviate this disadvantage and relates to a device in the above-described shadow microscope, by means of which the astigmatism of the electronoptical system can be observed and the visible astigmatism corrected. According to the invention, the X-ray tube of the X-ray shadow microscope comprises, in addition to the electron-optical system, a magnetic astigmatism-correcting element hereinafter referred to as magnetic stigmator and an electrostatic astigmatism-correcting element hereinafter referred to as electrostatic stigmator, the astigmatisms of which can be adjusted separately in value and direction and which are traversed both by electrons emerging from the cathode towards the focal spot and by secondary electrons emerging from the focal spot and travelling in the opposite sense, the electrostatic stigmator correcting the resulting astigmatism of the electron-optical system and of the magnetic stigmator.

The operation of the electron-optical system together with the two stigmators is based upon the fact that the astigmatism of ythe electron-optical system is different for the forward and backward rays. Consequently, if the energization is adjusted so that a small focal line lies on the target, such is not the case when this line is reproduced back. The focal line is thus reproduced with lack of definition and no longer recognizable as such due to the finite dimensions of the image proper. By means of the invention, at first the focal line is made visible and then the astigmatism corrected. For this purpose, advantage is taken of the fact that the effect of a magnetic stigmator inverses its polarity with the direction of movement of the electrons, whereas such is not the case with the electric stigmator. Consequently, for adjusting the corrected focal spot, that adjustment is chosen with the aid of the means arent for varying the value and direction of the astigmatism of the magnetic stigmator, for which the lack of definition in the image of a focal line falls exactly in the direction of this line. Subsequently, the astigmatism may be visibly corrected by means of the electrostatic stigmator.

In order that the invention may be readily carried into effect, it will now be described in detail, by way of example, with reference to the accompanying diagrammatic drawing showing the electron-optical device of the X-ray tube for use in an X-ray shadow microscope.

The wall of the X-ray tube is constituted by a glass tube 1 and a metallic cylinder 2. These parts are joined by sealing at 3. The base 4 of glass tube 3 has a reentrant part 5, into which fits the end 6 of a high-tension cable having current conductors 7 which are led through a transverse Wall 8 provided in re-entrant part 5, the Wall 8 closing the aperture and carrying a source of rays 9, constituted by a thermionic cathode, and a focussing elecfrode 10. The latter has a small aperture l1 to allow passage of the electrons emitted by the thermionic cathode.

The base 4 also contains a lead-through device l2 for a support 13 of a fluorescent screen 14 which is arranged inside the wall 8.

The axis of the beam of electrons and the perpendicular to the centre of the fluorescent screen are deflected from the axis of the X-ray tube at the same angles to meet at a point on this axis which is the centre of a magnetic transverse field having its lines of force at right angles to the plane in which the three directions are located. One of the poles of a magnet 15, between which the magnetic transverse field occurs, is shown. The pole which is not shown is located in front of the surface of the beam. The magnetic ltransverse field brings about deflection of the paths of the electrons emerging from the source of electrons and deflections, in the same sense of rotation, of the electrons travelling in the opposite sense.

The metal cylinder 2 has an end wall 16 provided with an aperture 17 which is closed by means of a thin metallic window 18. The cylinder 2 contains an electron-optical system which permits of focussing the paths of electrons into a very small focal spot on metal window 18. X-rays are emitted from this focal spot in all directions. The electron-optical system comprises three metal plates 19, 20, 21, which are arranged with small spacings. The plates are provided with apertures 22 23, 24concentrically of the axis of the tube, to allow passage of the ray of electrons. The outer plates 19 and 21 may be connected to the wall of the cylinder, plate 26 being insulated from the wall and the adjacent plates by means of a ring 26 of insulating material. It is possible to apply to plate 20 a potential suitable for focussing the rays of electrons by rneans of a connecting wire 25, which is led through lthe wall in an insulated manner.

Electrons return from the window in a direction opposite to that of the electrons travelling to the target through the electron-optical system and are concentrated onto the fluorescent screen 14.

If the electron-optical system has astigmatism, a corresponding image of that produced on metal window 18 is not obtained on fluorescent screen 14. For correction of the astigmatism, it is first necessary to obtain an image which is dependent upon the value and the direction of the astigmatism. This is possible with rods 27 of magnetic material. Both the forward and backward electrons traverse the field produced between the pole pieces. For their energization, the rods may be provided with energizing coils 28 arranged outside the Wall 2 of the X- ray tube around the magnetic rods. The rods 27 are magnetically separated from the wall by means of small rings 29 of non-magnetic material. Eight poles are provided in the magnetic stigmator so that the value and the direction of the added astigmatism are electrically adjustable. It is alternatively possible to utilize other known magnetic stigmators, for example a quadripole having poles which are rotatably arranged about the common axis.

A brightly illuminated spot on the luminescent screen indicates that the electron-optical system is exactly adjusted to the Window 18 and that the Surface between the source of rays and the system in which the beam has its smallest cross-section is reproduced. In the case of astigmatism, it is possible to modify the image on the fluorescent screen by energization of the magnetic stigmator and variation of the Value and directionV of the astigmatism thusV added. Proper adjustment of the s'tigmatorY makes the astigmatism of the electron-optical system visible when the image has its maximum deviation from the circular ashape. Next, the electrostatic stigmatoimay be switched on, which comprises eight electrodes 30 which surround symmetrically the axis of the electron beam and are se'- cured to supporting rods 31. The rods 31 also serve for the supply of the potentials and for this purpose are VledY in an insulated manner through the Wall 2 of the cylinder. Glass beads 32 sealed in the wall 2 may serve for insulation. The operation of such a stigmator is known and provides the possibility ofcorrecting the astigmatism made perceptible in the manner above described.

The invention is also applicable to an-X-ray tube comprising a magnetic electron-optical system, While in addition the two stigmators may be interchanged or positioned on the same side in the vicinity of the electron-optical system. These modifications do not inuence the elect envisaged by the' invention.

What is claimed is:

l. An X-ray shadow microscope comprising an envelope, an electron beam source within said envelope, a' target electrode, an electron-optical system for'focu'ssin'g electrons infsaid beam into a small focal spot on the target electrode for producing X-rays therefrom,` a uor'es'cent screen within said envelope and positioned to receive a beam of secondary electrons emitted from the focal spot, an" adjustable magnetic electron-beam astigmatism-correcting element disposed in the path of the electronbeam for partially correcting astigmatism in the electron beam produced by the electroni-optical focussingsystem, and an adjustable electrostatic electron beam astigmatism-correcting element disposed in the path of the electron beam for correcting the resultant astigmatism of the electron-optical system and the magnetic astigmatism-correcting element, the two astigmatism-correcting elements being positioned to correct astigmatism in the electron-beam traversing the electron-optical system and the beam of secondary electrons emitted from the focal spot. n v

2.V An X-ray shadowrmicroscope as claimed in claim l in which the electron-optical system comprises a plurality of plates provided with coaxial apertures concentric with the electron beam and means to apply potentials to lsaid plates to thereby focus the electrons inthe beam.

3. An X-ray shadow microscope as claimed in claim 2 in which the magnetic' astig'matism-correctingl element iS disposed between the electron-optical system and the target.

4. An X-ray shadow microscope as claimed in claim 3 in which the magnetic astigmatism-correcting element comprises a plurality of magnetic pole members surrounding the axis of the electron beam.

5. An X-ray shadow microscope as` claimed in claim 4 in which the pole members are ferromagnetic rods pro vided with energizing coils.

6. An X-ray shadow microscope as claimed inclaim 2 in which the electrostatic astigmatism-correctingmember is disposed between the electron-optical system and the uorescent screen. L*

7. An X-ray shadow microscope" as claimed-in claim 6 in which the electrostatic astigmatism-correcting member comprises la plurality of electrodes coaxial with and surrounding'the electron beam.

References Cited inthe' ile'of thispatent UNITED STATES PATENTS 

